1. Field of the Invention
This invention relates to the field of electro-magnetic communications, and in particular to a system for covertly modulating and demodulating an electromagnetic carrier wave so as to convey information from a transmitting device to a receiving device.
2. Description of Related Art
Modulation is the process of converting a message into information-bearing signals that not only unambiguously represent the message but also are suitable for propagation over a transmitting medium to a receiver. In pulse modulation, the carrier of the message is in the form of an electromagnetic wave which has been modulated to form a train of regularly recurrent pulses, the structure or arrangement of the individual pulses in the pulse train carrying the message. The transmitting medium may be the earth's atmosphere, outer space, or a man-made carrier such as an electrical or optical cable.
Conventionally, pulse modulation is accomplished by controlling the amplitude, duration, position, or mere presence of the pulses so as to represent the message to be communicated. These conventional forms of pulse modulation are commonly referred to as, respectively, pulse-amplitude modulation (PAM), pulse-duration modulation (PDM), pulse-position modulation (PPM), and pulse-code modulation (PCM).
PCM systems, in particular, are widely used in public and private communications by all branches of the military, Comsat, Intelsat, NASA, and private sector companies in virtually every country. By having to make only on or off distinctions, PCM is able to deliver a high quality signal even when noise and interference are so bad that it is barely possible to recognize the pulses, and thus PCM transmissions are especially suitable for the transmission of complex encoded messages. In addition, pulse communication systems, and especially PCM systems, have the advantages that the hardware and/or software required to send and receive such communications is generally simple, the resulting pulse trains are relatively easy to detect, and, once detected, the information content of a pulse train is readily extracted.
On the other hand, the ease of detection and data extraction advantages of pulse communications are disadvantages when the communications are intended to be covert. Encryption of the data can be used to make the communications and/or the content of the communications more difficult to detect by unfriendly parties, but the complexity of the system required to send and receive the messages greatly increases and, in the case of communications where the very existence of the communication, rather than just the information content, must be kept secret, conventional pulse communication systems are less than optimal. This is the case, for example, for communications during covert missions in unfriendly airspace.